1. Technical Field
This invention relates to control of plant parasitic nematodes, especially to a method of control of root cyst nematodes. This invention also relates to the introduction of nematode resistance into plants of a wide number of species which are susceptible to root cyst nematodes, for example potato plants (Solanum tuberosum)
2. Background Art
Cyst nematodes (principally Heterodera and Globodera spp) are key pests of major crops. They are responsible for direct loss in yield and also for indirect losses for instance due to the cost of pesticide and non-optimal use of land or rotation. Potato cyst nematodes (Globodera rostochiensis and Globodera pallida) are key pests of the potato in UK, many other parts of Europe and in other principal potato growing areas. Heterodera glycines (Soybean cyst nematode) has an economic effect on soybean that may exceed $500 m/y in USA alone. Heterodera schachtii (beet cyst nematode) is a major problem for sugar beet growers in Europe and USA. Heterodera avenae (cereal cyst nematode) has a worldwide economic status.
Economically significant densities of cyst nematodes usually cause stunting of crop plants. The root system is smaller than for uninfected plants, resulting in leaves showing symptoms of mineral deficiencies with an increased risk of wilting in dry soil conditions. Yield losses are related to the density of cyst nematode present at planting and in severe cases may be substantially above 50% for crops such as potato and soybean.
Current control depends upon chemicals, cultural techniques and resistant varieties all of which may be used in an integrated manner. Improved control is required. Nematicides are among the most unacceptable crop protection chemicals in widespread use. For instance Aldicarb and its breakdown products are highly toxic to mammals and have polluted groundwater. The result is that governments of several states within USA have placed restriction on the use of this nematicide. In The Netherlands there is a policy of reduction in nematicide use over a five year period. Cultural control is not an ideal solution to nematode control because measures such as crop rotation include hidden losses that are unacceptable to specialist growers or those with few economic alternative crops.
Resistance of a plant to nematodes may be effected by the inability of the nematode to reproduce on a genotype of a host plant species, and dominant, partially dominant and recessive modes of inheritance occur based on one to several plant genes. However, their commercial value is limited for the plant breeder and farmer. For instance, it has been observed in Europe that in potato different sources of resistance occur and create subdivisions of the nematode populations in Europe, as in the case of the single dominant gene HI which confers resistance against G. rostochiensis (Ro1 and Ro4) but not other forms of this species (Ro2, Ro3 and Ro5).
The cultivar Maris Piper expresses H1 and is widely used in the UK against G. rostochiensis but its use has been correlated with an increased prevelance of G. pallida which is able to reproduce in the presence of H1. The problem posed by resistance-breaking pathotypes occurs for other cyst nematodes and sources of resistance. In some cases, sources of resistance are polygenic, presenting more difficulties for the plant breeder. In addition the resistance in some cultivars is quantitative rather than qualitative in nature and so increased nematode multiplication may occur with time in response to frequent use of such cultivars.
The interface between plant and pathogen is a site of key importance in the determination of susceptibility or resistance to invasion. During early determinative stages of the invasion process, the interface is restricted to a very small number of plant cells at the local site of infection. Later redifferentiation of existing plant cells forms a syncytium from which the animal feeds. The syncytium is induced by second-stage juveniles after they have migrated into the roots of a host probably in response to secretion. released by the animal into an initial feeding cell. When the plant is susceptible, the syncytium increases in volume and is maintained throughout the feeding period of the nematode. In the case of females this may occupy several weeks. The cell biology of the syncytium is well characterised.